Jornal de Pediatria - Vol.79, Supl.2, 2003
S153
of sufficient PEEP or both (Figure 3). However, strategies
that apply sufficient PEEP while avoiding alveolar over
distension can prevent the generation of pro-inflammatory
mediators (biotrauma) that may adversely affect the
progression of the pulmonary lesion,
17,20
as well as damage
remote organs if these substances were to enter into
circulation.
21
Despite the protective role of PEEP having
been systematically documented in laboratory studies, the
North American multi-center clinical trial of patients treated
with a high pulmonary expiratory volume and low FiO
2
compared with patients treated with a low pulmonary
expiratory volume and high FiO
2
was recently terminated
due to futility after the inclusion of 550 patients.
22
ARDS who develop significant hypercapnia when protective
ventilation is initiated (elevated PEEP with limited Vt) are
promptly started on high frequency oscillatory ventilation.
Ventilation mode
Modern conventional mechanical ventilators offer an
increasing array of ventilation modes for use in patients
with ARDS. Conceptually, however, most ventilation modes
used in ARDS are similar in that they are cycled by time and
limited by volume or pressure. A mode that is cycled by time
and limited by volume implies that the cycle (inspiration
and expiration) is controlled by time (inspiratory time and
breath rate), and that during the inspiratory phase of the
cycle a certain pre-determined volume is administered. A
mode that is cycled by time and limited by pressure implies
that the cycle (inspiration and expiration) is controlled by
time (inspiratory time and breath rate), and that during the
inspiratory phase of the cycle a certain pre-determined
pressure is administered. In volume-limited ventilation, the
Vt administered during each inspiration generates a certain
airway pressure (which is measured and controlled in current
ventilators). Similarly, in pressure-limited ventilation, the
application of a specific pressure gradient between the
ventilator and the airway results in the generation of a
certain Vt that can be measured and controlled. Regardless
of the ventilation mode used, it is important to emphasize
that no one conventional ventilation mode has been shown
to be clinically superior to another in the management of
patients with ARDS, as long as the principles of protective
ventilation are respected.
Considering that precise Vt control is a very important
factor in ARDS support, time-cycled volume-limited modes
are preferred by the most of intensive care specialists
nowadays. In time-cycled volume-limited ventilation
(controlled, assist-controlled, intermittent mandatory or
intermittent mandatory with pressure support) the operator
defines the exact Vt to be administered by each mandatory
ventilator cycle. The pressure measurements generated by
this set volume at the end of inspiration (dynamic) or after
a pause (static or plateau pressure) are indicators of
pulmonary compliance in ARDS. A peak inspiratory pressure
which increases over time for a fixed volume generally
indicates worsening compliance. In an analogous manner,
a reduction in peak inspiratory pressure generally indicates
an improvement in compliance. Volume-limited ventilation
traditionally generates a triangular pressure waveform, in
contrast with the rectangular waveform of pressure-limited
ventilation (Figure 4). As the area under the pressure curve
reflects mean airway pressure, volume-limited modes
(triangular waveforms) generally have a slightly lower
mean airway pressure than pressure-limited modes
(rectangular waveform). Modern ventilators like the Servo
300, however, offer a mode known as pressure regulated
volume control (PRVC), in which the shape of the pressure
waveform of this volume-limited mode is similar to the
rectangular format of the pressure-limited mode. As such,
Acute respiratory distress syndrome – Rotta AT
et alii
Figure 3 - Comparison of PEEP and tidal volume (Vt) among
different randomized controlled studies of reduced
tidal volume strategies in ARDS
Amato
5
19
18
15
Stewart
Brochard
ARDS Network
0
5
5
5
5
0
0
0
10
PEEP (cm H O)
2
VT (ml/kg)
VT (ml/kg)
VT (ml/kg)
VT (ml/kg)
PEEP (cm H O)
2
PEEP (cm H O)
2
PEEP (cm H O)
2
10
10
10
10
10
10
10
20
15
15
15
15
20
20
20
In clinical practice, pediatric patients with ARDS should
be ventilated with PEEP that is capable of maintaining
adequate pulmonary volume at the end of expiration. This
value is generally above 8 cm H
2
O and below 20 cm H
2
O,
other than in exceptional cases. Positive end-expiratory
pressure should be progressively increased (in 2 to 3 cm
H
2
O increments) to optimize oxygenation (saturation
between 90 and 95% with FiO
2
< 0.5) and pulmonary
inflation checked with chest radiographs or computerized
tomography. Patients with severe anasarca or other restrictive
lesions of the chest (circumferential burns), as well as
patients with excessive abdominal pressure, may require
higher PEEP levels.
A strategy that limits Vt while at the same time applies
an ideal PEEP, generally results in a reduction in minute
volume and hypercapnia, even when the respiratory rate is
increased.
17
Strategies with permissive hypercapnia
(controlled hypoventilation), in which an elevation in PaCO
2
up to approximately 80 torr is accepted as long as pH is kept
above 7.25, are well tolerated by adults
23
but may have
adverse effects
24
and have not been adequately tested with
children. In our clinical practice, pediatric patients with